1. Field of the Invention
The present invention relates to a mixer circuit used, for example, for an LSI (Large Scale Integrated circuit) for radio communication.
2. Background Art
Conventionally, there has been used a mixer circuit which performs frequency conversion of radio frequency (RF) signals in radio communication.
The mixer circuit includes an active mixer of a type that receives an input signal as a current and switches current paths by turning on and off an MOS switch, into which a local oscillation (LO) signal is inputted By the switching of current paths, current signals having frequencies of the sum and difference of the frequency of the input voltage signal and the frequency of the LO signal are outputted to an output terminal.
In this active mixer, when a steady state current flows to the above described MOS switch constituting the mixer circuit, a flicker noise proportional to 1/frequency is generated.
Particularly, in the case where the mixer circuit is a down conversion mixer which is used for a direct conversion mode receiver or a Low-IF (intermediate frequency) mode receiver, this flicker noise becomes a problem.
In order to solve this problem, a passive mixer in which the steady state current is not permitted to flow is adopted as the mixer circuit. Generally, in the passive mixer, for example, a suitable potential obtained by dividing a power supply voltage with the use of a common mode feedback technique, is given as the source potential and the drain potential of the MOS switch, respectively. Furthermore, the gate potential of the MOS switch is set, for example, to the other potential obtained by dividing the power supply voltage by resistors.
However, the passive mixer generally needs a local oscillation signal having a large amplitude in order to obtain a sufficient conversion gain, which results in an increase in current consumption.
Furthermore, in the case where the bias potential of the above described MOS switch is set to a fixed potential generated by dividing the power supply voltage by resistors, as described above, when a threshold voltage is lowered due to process variation, temperature variation and power supply voltage variation, the above described MOS switch is brought into on-state in a steady state so as to permit a steady state current to flow, as a result of which the flicker noise is generated. Therefore, even when the passive mixer is adopted, the noise characteristic is deteriorated due to process variation, temperature variation, and power supply voltage variation.
Here, the conventional mixer circuit includes, for example, a mixer circuit having a bias circuit for supplying a bias voltage applied to the gate of an MOS switch, into which an IF signal is inputted This bias circuit outputs the bias voltage by using a differential amplifier on the basis of a voltage at which the MOS transistor as a component operates in a pentode region (see, for example, Japanese Patent Laid-Open Publication No. 2005-184141).
The above described conventional semiconductor integrated circuit, as described above, controls the gate voltage of the MOS switch into which the IF signal is inputted, and does not control the gate voltage of the MOS switch into which the local oscillation signal is inputted.
Furthermore, in the above described conventional semiconductor integrated circuit, the voltage applied to the source and the drain of the MOS switch into which the local oscillation signal is inputted, is not referred to. Thus, even when the bias voltage is controlled by the above described bias circuit, the threshold value of the MOS switch varies due to process variation, temperature variation, and power supply voltage variation.
That is, even with the above described conventional semiconductor integrated circuit, there is a problem that the flicker noise as described above may be generated due to process variation, temperature variation, and power supply voltage variation, to thereby cause the noise characteristic to be deteriorated, while increasing the conversion gain of a mixer circuit.